Drug delivery systems for delivering drugs to a suitable site in the body is researched and developed as providing new pharmaceutical agents having limited side effects. Among these systems, drug delivery systems using polyion complexes (hereinafter also referred to as “PIC”) have attracted attention as techniques enabling drugs to be specifically delivered to affected parts by including the drugs in nanomicelles. The “polyion complex” generally means an ion layer obtained by mixing a copolymer of PEG and a cationic block and a copolymer of PEG and an anionic block in a solution, which ion layer is then formed between the cationic block and the anionic block, between the block copolymers. The linkage of the charged chain to PEG suppresses the aggregation and precipitation of the polyion complex and facilitates the formation of a nanoparticle having a monodisperse core-shell structure with a particle diameter of a few tens of nm. At that time, PEG covers the shell of the nanoparticle, which is known to be convenient in terms of increasing biocompatibility and enhancing blood residence time.
As techniques to which PIC is applied, systems have particularly been developed which can deliver nucleic acids into the body with limited side effects (Patent Literatures 1 and 2). Patent Literatures 1 and 2 propose polyion complexes of DNA and new cationic polymers.